1. Field of the Invention
The present invention relates to a non-magnetic toner, a two-component developer and an image forming apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus comprises image forming process mechanisms such as a photoreceptor, a charging section for charging the photoreceptor surface, an exposure section for irradiating the charged photoreceptor surface with signal light to thereby form thereon an electrostatic latent image corresponding to image information, a developing section for supplying a toner in a developer to the electrostatic latent image on the photoreceptor surface to thereby form thereon a toner image, a transfer section for transferring the toner image from the photoreceptor surface onto a recording medium, a fixing section for fixing the toner image on the recording medium, and a cleaning section for cleaning the photoreceptor surface after the toner image transference, in which the electrostatic latent image is developed with a one-component developer that contains a toner alone or with a two-component developer that contains a toner and a carrier, thereby forming an image.
Such an electrophotographic image forming apparatus may form a good image of high quality at high speed and inexpensively, and is therefore utilized in duplicators, printers, facsimiles, etc., and the spread of the apparatus is remarkable these days. With that, the requirements of image forming apparatus are being much severer. In particular, high definition and high resolution of images to be formed, stabilization of image quality and high-speed image formation in image forming apparatus are regarded as important. To attain these, investigation of both an image forming process and a developer is indispensable.
Regarding the requirement for high-definition and high-resolution images in terms of the developer to be used for image formation, one problem to be solved is how to reduce the size of toner particles from the viewpoint that faithful reproduction of electrostatic latent images is important, and various proposals have been made for it.
For example, a non-magnetic toner is proposed, of which the particle size distribution satisfies the following: The content of the toner particles having a particle size of at most 5 μm is from 17 to 60% by number, the content of the toner particles having a particle size of from 8 to 12.7 μm is from 1 to 30% by number, the content of the toner particles having a particle size of at least 16 μm is at most 2.0% by volume, the volume-average particle size is from 4 to 10 μm, the toner particles having a particle size of at most 5 μm satisfies N/V=−0.04N+k (wherein N indicates % by number of the toner particles having a particle size of at most 5 μm, and is a positive number of from 17 to 60; V indicates % by volume of the toner particles having a particle size of at most 5 μm; k indicates a positive number of from 4.5 to 6.5) (e.g., see Japanese Unexamined Patent Publication JP-A 2-877 (1990)). The technique in JP-A 2-877 is to overcome the drawback of the toner particles having a particle size of at most 5 μm in that, since the electric field intensity at the edges of an electrostatic latent image is higher than that in the center thereof, the amount of the toner particles to adhere to the center part of the electrostatic latent image is smaller than that to adhere to the edges thereof whereby the image density lowers, by specifically defining the content of the toner particles having a particle size of at most 5 μm to be within a specific range. The non-magnetic toner in JP-A 2-877 may be advantageous for high-definition and high-resolution image formation, but its flowability is poor, and therefore scattering of the toner may tend to soil machines. In particular, in a low-humidity environment, the toner particles may be overcharged (charge-up), and the overcharged toner particles may firmly adhere to the carrier surface in a developer and to the photoreceptor surface thereby causing image fogging, photoreceptor cleaning failure, and filming on photoreceptor, and therefore detracting from the durability of both the image forming process and the developer. In addition, the toner particles may readily form their aggregates to cause white skip in images.
A toner is also proposed which has a mean particle size of from 5 to 10 μm and in which the content of the toner particles having a particle size of at most 5 μm is at most 10% by number (e.g., see Japanese Unexamined Patent Publication JP-A 10-91000 (1998)). In the toner, the content of the toner particles having a particle size of at most 5 μm is too small, and therefore the toner is unsuitable for high-definition and high-resolution image formation.
Also proposed is a toner comprising toner particles that contain a binder resin and a colorant, and an external additive added thereto, wherein the toner particles satisfies 1.45−0.05D50≦D25/D75≦1.75−0.05D50 (in which D25, D50 and D75 each indicate the particle size where a cumulative volume from the large particle size side in a cumulative volume distribution of the particles reaches 25%, 50% and 75%, respectively), D50 is from 3 to 7 μm, the external additive comprises hydrophobic inorganic particles having a number-average particle size of from 5 to 70 nm and inorganic particles having a number-average particle size of from 80 to 800 nm in which the content of the particles having a particle size of at least 1000 nm is at most 20% by number (e.g., see Japanese Unexamined Patent Publication JP-A 10-207112 (1998)). The toner has good flowability, prevents image fogging, and has good photoreceptor cleanability, but is not still effective for high-definition and high-resolution image formation.
Also proposed is a toner of such that the content of the toner particles having a particle size of at most 5 μm is at most 15% by number, the toner has a weight-average particle size of from 6.0 to 11.5 μm, the content of the toner particles having a particle size of at least two times the weight-average particle size thereof is at most 5% by volume, and the ratio of the number-average particle size D25 to D75 (D25/D75) where a cumulative number distribution of the toner reaches 25% and 75%, respectively is from 0.60 to 0.80 (e.g., see Japanese Unexamined Patent Publication JP-A 2005-43918). The toner also has good flowability, prevents image fogging, and has good photoreceptor cleanability, but is ineffective for high-definition and high-resolution image formation, like the toner in JP-A 10-207112.